Replacing a missing/obsolete AC power supply with DC power supply

Question

I am trying to replace a missing power supply for a TRS-80 model 1. The original power supply provided unregulated DC ~20V, unregulated AC ~10V and ground. Internally the computer connects the AC to a full wave rectifier with smoothing capacitors. TRS-80 model 1 technical manual, schematic is on page 37 and 38

My understanding is that if I supply positive and negative DC to the AC lines and account for the voltage drop through the rectifier, I should be ok. I would like to keep the original computer unmodified, if possible.

Answer 1

From the circuit it seems AC input is used to provide +/-5V rails, and capacitors on those inputs are rated for 16V. Also note that the ground is part of AC supply too. It is not single 20VAC power, it is 2 x 10VAC supplies.

I'd suggest either using center-tapped 2 x 9-10V transformer from some old device, if you can find one, or two 10-12V DC adapters.

The DC input is regulated to +12V and it has capacitor rated for 35V. Third DC supply of about 15-20V should be OK for this.

So, 3 DC adapters in total. Or you can find transformer with 3 secondary coils, rectify one in your adapter and connect them together for common ground.

UPDATE

After some digging on the web found this. It confirms my guess above how the whole thing was wired, although I did not expect two transformers in there. Good to know exact expected voltages too.

Answer 2

If the AC power supply starts with a full wave bridge, then yes, you can feed it DC instead of AC. However, there are some gotchas:

1. The DC should be sqrt(2) of the AC RMS value. That's what the peaks of the AC were, which is the voltage the capacitor after the full wave bridge got charged to.

2. The DC power must be floating. Neither input to a full wave bridge is ground. There might simply be a diode internally between each input and ground, but unless you know that's really true, you must treat the AC input as floating.

What this means for you in practice is that you need two supplies that are independent of each other. Fortunately, power supplies are cheap and plentiful nowadays, so this really not much of a problem. Ideally you want one to be 20 V, and the other 28 V. However, there is probably considerable slop in the voltage range of these inputs, so I'd try two 24 V power supplies. There is a good chance that will work fine.

Connect the two supplies to the same AC feed on the input end, and connect their outputs to the two separate power inputs of your unit.

You'll need to have some idea of the power requirement before you go out and buy supplies. Get supplies that meet or exceed the power requirement.